A continuous underground wall construction method is known for constructing walls for blocking soil during the construction of underground structures and for constructing walls for blocking water to be constructed around a dam or a waste disposal facility. In this type of construction method, a rectangular ditch extending from the surface of the soil foundation up to a certain depth is dug, and a hardening substance such as concrete is then filled into the ditch to thereby form an unit wall in panel shape.
By repeating such process in turn, a continuous underground wall can be constructed in the soil foundation by continuously forming the unit wall in a row in the horizontal direction. In this method, there is a necessity to dig the soil foundation for making the rectangular ditches in the soil foundation. As an apparatus for digging soil foundation to be used for this type of digging, for example, grab-type digging apparatuses or rotary cutter-equipped digging apparatuses have been normally employed.
However, the prior digging apparatuses of such types have a technical problem as described hereinbelow.
Except in the case that the underground wall to be constructed is used as a part of an objective structure, it is enough for the underground wall if it has a strength required for a wall as far as the underground wall is used as a temporary structure. However, it is hard for the grab-type digging apparatuses and the rotary cutter-equipped digging apparatuses of the past to make relatively thin ditches in soil foundation. Although the miniaturization of these apparatuses can facilitate to make such thin ditches to come extent, it was very difficult to make thin ditches in a soil foundation particularly a soil foundation which contains a lot of stone pieces, even by using such miniaturized-apparatus. Therefore the construction of a thin underground wall in soil foundations containing a great amount of stone pieces was almost impossible to carry out.
On the other hand, an apparatus for digging a soil foundation wherein a plurality of cylindrical digging bits are linearly arranged has been provided for the construction of columnar under ground walls in a row. With this type of apparatus for digging in soil foundation, it is possible to make relatively thin ditches in a soil foundation even it contains lot of stone pieces if the diameter of the digging bit is reduced and a device for impressing percussion force is used. However, when a plurality of cylindrical digging bits with a small diameter are arranged in a row in the apparatus, the structure of the apparatus become much too complex and then the possible number of digging bits to be arranged is limited. Thereby the length of the ditch made at one time of digging is limited to short lengths, which leads to a problem of the decrease in the efficiency for the digging.